Austenite formation during intercritical annealing
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I. INTRODUCTION
THERE is currently a large interest in producing steels with superior mechanical behavior as, e.g., provided by dualphase (ferrite/martensite) and transformation-induced plasticity (TRIP) steels. To generate the required microstructures, a processing route can be used, which involves cold rolling of ferrite/pearlite microstructures followed by intercritical annealing on a continuous annealing line, usually a hot dip galvanizing line. This processing route can be used to produce multiphase steels such as dual-phase (ferrite/martensite) and TRIP steels; the latter have a substantial fraction of retained austenite (5 to 20 pct). Although a general understanding of these materials has been developed, a number of fundamental issues remain to be solved. Furthermore, experience has shown that these steels are considerably more sensitive to production variations compared to conventional steels. A careful control of processing conditions and the identification of critical processing steps is crucial to the success of these products. The challenge, therefore, is to develop industrially relevant processing routes that will lead to the desired properties with a minimum of variation. The processing routes for cold-rolled and annealed dualphase and TRIP steels involve reheating the steel into the intercritical region, where an austenite/ferrite mixture is formed. For dual-phase steels, cooling to room temperature produces a microstructure consisting of ferrite and martensite. The cooling path and composition of these steels must be carefully optimized to minimize transformation back to ferrite or the pearlite and bainite reactions. This is usually facilitated by alloy additions such as Mn, Mo, or Cr. In the case of TRIP steels, cooling followed by holding at an intermediate temperature in the bainite transformation range is required to careJ. HUANG, Graduate Student, W.J. POOLE, Associate Professor, and M. MILITZER, Associate Professor, are with the Department of Materials Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Contact e-mail: [email protected] Manuscript submitted March 24, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
fully control the decomposition of austenite into bainite and martensite, with some austenite retained in the final microstructure. Again, alloying additions play a critical role, in this case, usually a combination of Mn, Si, and more recently Al. The addition of Si or Al is required to minimize carbide precipitation, thereby enabling austenite retention at room temperature. The mechanical properties of the dual-phase and TRIP steels are primarily dependent on the volume fraction, carbon concentration, and distribution of the final transformation products,[1,2] which inherit the composition and distribution of the parent austenite phase. Therefore, the austenite condition at the intercritical temperature is important for the evolution of the final microstructure of the steel and consequently its mechanical properties. The formation of austenite
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